This invention relates to a thermal switch for detecting an abnormal temperature rise of, for instance, fire detectors or electric heating appliances.
In conventional types of fuses widely used, a band-shaped metallic conductor of a relatively low fusing point such as lead is employed as a fusible member and terminals are attached to both ends of the band-shaped fusible member for attachment to connecting portions of a circuit. A weight is attached to the fusible member at the center thereof and when the fusible members starts to fuse, the weight falls off the fusible member due to its weight to rapidly divide the fusible member to cut off the circuit, thereby actuating a fire alarm or automatically cutting off the power supply to an electrical heating appliance.
Since the fusing point of the abovesaid metallic conductor is 110.degree. C. or higher at the lowest, it is impossible to provide a fuse which operates at a temperature below the above-said one. Accordingly, where such a metallic conductor is used in a fire detector, a warning cannot be immediately given. In addition to this, the surface of the fuse of such a metallic conductor is readily oxidized to form thereon an oxide film. Accordingly, there is the possibility that even when the fusing temperature is reached, only the inner part of the fuse is fused but that its surface remains unfused. In such a case, the circuit is not cut off, which entails a danger such as a delay in the detection of a fire or overheating of an electric heating appliance. Further, in the case of a fuse formed of an alloy which is composed of lead and, for example, tin or the like, for the purpose of lowering the fusing temperature, the mixed component may be evaporated in some cases, with the result that the mixed component decreases to raise the fusing temperature. For instance, if a fuse having a fusing temperature of 110.degree. C. is left, for example at 90.degree. C. for ten days, the fusing temperature sometimes varies from 110.degree. C. to 160.degree. C. To avoid this, there has been proposed a thermal switch of the type which employs an insulating fusible powder having a low fusing point such, for example, as paraffin, and which is adapted so that contactors are prevented by the fusible powder from movement to maintain the circuit in its conducting state and that when the fusible power is fused by a temperature rise, the contactors shift to cut off the circuit. With the use of paraffin, it is possible to obtain a fire detector which operates at a relatively low temperature, for instance, about 50.degree. C., and which has the advantage of detecting a fire at its early stage.
The prior art thermal switch employing an insulating fusible powder such as paraffin comprises an insulating cylindrical member fabricated of a material such as glass or ceramics, conductive caps attached to both ends of the insulating cylindrical member, three cup-shaped conductors disposed in the insulating cylindrical member to electrically interconnect the conductive caps and disconnect them from each other when the insulating fusible powder is fused, a first bias spring for holding the three contactors in contact with one another to retain the conductive caps in their electrically conductive state when the insulating fusible powder remains unfused, and a second bias spring for bringing the contactors out of contact with one another when the insulating fusible member is fused.
With such a structure, it is possible to obtain a thermal switch which employs a small insulating cylindrical member about 6 mm in diameter and about 12 mm in length and has a current capacity of approximately 10 amperes. However, this thermal switch has the defect that the operation of incorporating the three cup-shaped contactors in the insulating cylindrical member is very troublesome. Further, the use of the three contactors increases the number of contact points, which leads to the defect that their contact resistances increase the resistance value between the conductive caps.
One object of this invention is to provide a thermal switch of the abovesaid type employing an insulating fusible powder and in which the number of cup-shaped contactors mounted in the insulating cylindrical member is reduced to simplify the assembling operation and decrease the contact resistance between the conductive caps.
Another object of this invention is to provide a thermal switch which ensures disconnection of the contactors when the insulating fusible powder is fused.
Another object of this invention is to provide a thermal switch wherein the range of dispersion of the operation temperature is narrow.
Still another object of the invention is to provide a thermal switch which has an excellant moistureproof characteristic and whose operating temperature does not change with the lapse of time.